The present invention relates to a semiconductor device manufacturing method. More particularly, the present invention relates to a method for forming a MOS transistor having a bi-layered spacer.
As semiconductor devices become more highly integrated, there is a need to decrease the size of MOS transistors used for semiconductor devices. This reduction in the size of MOS transistors causes a short channel effect. The short channel effect lowers the reliability of the MOS transistor, due to a hot carrier effect, and degrades the electrical characteristics of the MOS transistor e.g., leakage current characteristics between the source and drain regions. As a result, MOS transistors have increasingly adopted the use of a lightly doped drain (LDD). In order to form a LDD region, it is necessary to form spacers at the side walls of a gate electrode. Also, the gate electrode of the MOS transistor must be formed of a conductive material film containing a metal silicide film having a low resistivity, in order to improve the operation speed of a semiconductor device.
FIGS. 1 through 3 are section views illustrating a related method for forming a MOS transistor.
Referring to FIG. 1, a gate dielectric film 3 is formed in a semiconductor substrate 1, and a polysilicon film 5, a titanium silicide film 7 and a gate protection film 9, which form a gate pattern 10, are stacked in sequence on a predetermined region of the gate dielectric film 3. The gate protection film 9 is formed as a silicon oxide film or silicon nitride film.
Referring to FIGS. 2 and 3, a silicon nitride film 11 having a predetermined thickness is formed over the entire surface of the semiconductor substrate 1 having the gate pattern 10. Preferably, the silicon nitride film 11 is formed to have a dense film quality in order to increase etching selectivity with respect to other dielectric films such as a silicon oxide film. Thus, the silicon nitride film 11 is formed at a high temperature of 750.degree. C. to 800.degree. C. Here, during the formation of the silicon nitride film 11, the titanium silicide film 7 is easily oxidized or deformed. Thus, a deformed titanium film 7a as shown in FIG. 2 is formed and simultaneously an undesired dielectric film 7b such as titanium oxide film is formed at the side walls of the deformed titanium silicide film 7a.
As a result, the width of the deformed titanium silicide film 7a, which actually contributes to the transfer of an electrical signal is decreased to less than the width of the initial titanium silicide film 7. As a result of this, the resistance of a gate electrode including the polysilicon film 5 and the deformed titanium silicide film 7a increases. Then, the silicon nitride film 11 is anisotropically etched to form spacers 11a at the side walls of the gate pattern 10.
According to the conventional method described above, the silicon nitride film for forming the spacers is formed at a high temperature of 750.about.800.degree. C., so that the titanium suicide film of the gate electrode is deformed. As a result, the resistance of the gate electrode increases, thereby degrading the operation characteristics of the semiconductor device. Such a phenomenon is serious in a MOS transistor used for a highly integrated semiconductor device, because of the short channel length of the MOS transistor, i.e., the narrow gate electrode of the MOS transistor.